Arguments and values to get.

[Kent M Pitman <KMP@SCRC-STONY-BROOK.ARPA>]

    Date: Thu, 19 Sep 1985  21:19 EDT
    From: "Scott E. Fahlman" <Fahlman@C.CS.CMU.EDU>
    To:   Gregor.pa@XEROX.ARPA
    In-reply-to: Msg of 19 Sep 1985  19:39-EDT from Gregor.pa at Xerox.ARPA

	Why does Common-Lisp GET take a default-value argument instead of
	returning a second value indicating whether the property actually
	exists?

    The default argument is useful in some cases, but admittedly is a bit
    inelegant -- if you want to know for sure whether a value was found, you
    have to be careful to choose some default argument that couldn't
    possibly be stored in any property list.  The reasoning (If I recall it
    correctly) was that this occasional creation of a unique default is less
    hassle than having to write an ugly multiple-value-bind every time we
    want to return a default value in the property-not-found case.  I'm not
    sure I believe that any more.

    -- Scott

It would be reasonable to propose a more-or-less upward-compatible extension 
to GET (for CL86 or whatever) which provided that GET both accepted a argument
specifying the default to be used -and- returned two values (one the result, 
the second a boolean saying whether the argument had been defaulted).